1. Field of the Invention
A large number of oxidation catalysts have been suggested, and their use for the preparation of methacrylic acid by gas-phase oxidation of methacrolein has been proposed. However, these catalysts satisfy only some, if any, of the requirements for industrial operation in respect of formation of a small amount of by-products, in particular of acetic acid, maleic acid and citraconic acid, coupled with a technically reasonable size for the catalyst particles and high space velocities with respect to methacrolein.
2. Description of the Prior Art
U.S. Pat. No. 3,772,381 discloses, for example, catalysts which contain Mo, Cu, P, Sb and Cs and/or Ca, but, in the formation of methacrylic acid, these catalysts give an unsatisfactory selectivity of 76% at a methacrolein conversion of 75%. German Laid-Open Application DOS No. 2,523,757 proposes catalysts which, in addition to Mo, Cu and P, contain one or more alkali metals and one or more metals selected from the group comprising Sb, V, W, Fe, Mn and Sn. Although long-term operation with these catalysts results in methacrolein conversions as high as 91.5% and selectivities of 82%, the low space velocity of 1000 h.sup.-1 and the relatively high temperatures of 325.degree. C. and higher are unsatisfactory for industrial operation. Oxidation catalysts which are of the type disclosed in British Patent No. 2,046,252 and contain Mo, P and V and may or may not contain As and Cu or other cationic elements also exhibit high catalytic activity, but only for sizes of the catalyst particles of less than 2 mm which are not relevant industrially, and at a relatively high temperature of 330.degree. C. Oxidation catalysts which are prepared in the presence of a high chloride ion concentration of about 1-5 equivalents per equivalent of molybdenum, for example the catalysts which contain Mo, P and W and are described in European Patent No. 00 10 429, or those which contain Mo, P and Sb and may or may not contain W and are described in U.S. Pat. No. 3,965,163, exhibit relatively good catalytic activity during short operating times; however, catalysts of this type which have a long life and are sufficiently selective are difficult to prepare in a reproducible manner. Furthermore, the stated catalysts have a tendency to form larger amounts of acetic acid when they are used in industrially reasonable particle sizes of 3 mm or more. British Patent No. 2,001,256 discloses other oxidic catalysts which contain Mo, P, As, Cu and Cr and are prepared in the presence or absence of a dibasic carboxylic acid, hydroxycarboxylic acid, mannitol or pyrogallol as a reducing agent. However, the properties of these catalysts and of the abovementioned ones are generally unsatisfactory when the raw material used for the preparation of methacrylic acid is methacrolein which has been prepared by condensation of propanol with formaldehyde. As a result of the preparation process, this methacrolein is contaminated with unreacted propanol as well as organic amines, dimers of methacrolein and methyl pentenal. Even small amounts of these impurities generally result in a reduction in the performance of such catalysts.